<!DOCTYPE html>
<html lang="zh-Hans">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>animation</title>
  <style>
    .common {
      width: 100px;
      height: 100px;
    }

    #el_1 {
      background-color: lightblue;
    }

    #el_2 {
      background-color: lightgreen;
    }
  </style>
</head>
<body>
  <div class="common" id="el_1"></div>
  <div class="common" id="el_2"></div>

  <button id="pause">pause</button>
  <button id="resume">resume</button>

  <script src="animation-demo.js"></script>
</body>
</html>